multiplexed energy metering afes ease asic integration and provide significant cost reduction
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7/27/2019 Multiplexed Energy Metering AFEs Ease ASIC Integration and Provide Significant Cost Reduction
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Multiplexed Energy Metering AFEs Ease ASIC Integration and Provide Significant Cost
ReductionBy Christian Domingues, Analog Designer, Dolphin Integration June 2010 GSA Forum
Smart electric meters are fundamental to the successful deployment of smart grid technology, as they improve grid reliability and use
consumption control and reduce electricity theft. The variety of consumers emerging needs requires a much wider offering of energy metering
systems-on-chip (SOCs), paving the way for more fabless companies to enter the energy measurement field. Energy meter-specific analog front
end (AFE) devices, which combine high performance with cost reduction, are thus needed to complement standard IC offerings.
Todays energy metering standards demand higher accuracy and lower power consumption which, in turn, challenges system designers to delivemore competitive AFEs. This article reviews those challenges and presents a solution based on a multiplexed channel architecture that delivers
ultra-high resolution, along with very low-power consumption and silicon area. First, the article gives an introduction to smart electric meters and
their specifications. Second, it presents the architecture used in conventional energy meter AFEs, and compares the trade-offs of using a high
performance analog-to-digital converter (ADC) versus using a lower performance ADC together with a programmable gain amplifier (PGA). Third, a
new multiplexed AFE architecture for three-phase energy meters, which yields considerable area and power savings while simplifying the
integration of application-specific ICs (ASICs), is detailed. Finally, the need for multi-domain simulation to guarantee AFE performances at the
system level is discussed.
Smart Electric Meters
Electric meters, also called Ferraris e-meters, are simple metal disks rotating in a magnetic field due to induced currents. They were first
introduced in residential houses at the beginning of the 20th century and were used until the last decade when the electricity industry started to
adopt electronic meters. Smart electric meters are beginning to replace the old meters because they offer higher accuracy and require less power
at a considerably lower cost. Furthermore, they offer additional functional benefits such as real-time reading, tampering detection, remote reading
and service power outage notification.
Figure 1 shows the architecture of an application-specific standard product (ASSP) for single-phase energy meters with tampering detection, which
is used by many manufacturers. The ASSP contains an AFE to convert the analog input signal given by current and voltage sensors into digita
information. Digital signal processing is used to compute the different energy metrics, such as instantaneous, active and reactive power;
voltage/current value; and power factor. A micro controller unit (MCU) manages the system and its peripherals (e.g., real-time clock, liquid crystal
display (LCD), communication ports/modules). Current transformers (CTs), resistive shunts and Rogowski coils can be interfaced to the AFE to
measure current, while resistive bridge and voltage transformers are used to measure voltage.
Figure 1. System Block Diagram for a Single-Phase Energy Meter
Standard Compliance
Energy meters are specified according to their class and range which are defined by European International Electrotechnical Commission (IEC) and
American National Standards Institute (ANSI) standards. Its class refers to the accuracy of measure, and its range refers to the dynamic range across
which accuracy should be achieved. Each standard also specifies environmental requirements, such as how much power the meter itself can
dissipate and how much voltage it must tolerate. Classes between 0.1-2 meters with ranges between 500-3,000 and greater are available today.
From the Energy Meter Class/Range Specification to the AFE Performance Specification
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The AFE is critical in meeting application objectives since it provides the link between the real world and the processing world. To ensure standard
compliance, the SOC integrator must be able to translate the class/range specification to the AFE fundamental requirements commonly known as
signal-to-noise ratio (SNR), input referred noise voltage or equivalent number of bits (ENOB). An energy meter with a class of 0.1 and a range o
1/1,000 must measure active power with less than 0.1 percent error over a current variation of 1,000 to 1 or better. For such a class/range target,
the ADC should resolve a minimum detectable signal of 1 Vrms over a dynamic range of 1 Vrms, which requires it to have a SNR of 120 dB or an
ENOB of 19.6b. It is also important to understand that the AFE error budget is much less than 0.1 percent in relation to the total error meter
budget. Assuming a CT with 0.07 percent accuracy requires an AFE accuracy of 0.07 percent with a 123 dB SNR ADC.
Conventional AFE Architecture Used in Energy Metering
As previously discussed, an energy meter with a class of 0.1 and a range of 1/1,000 requires an AFE with a resolution of 19.6b. One approach usedto achieve this class/range specification uses a high-performance ADC with accuracy better than 19.6b. Such virtual component (ViC) ADCs are
available, but are not cost-competitive for energy meter applications. Figure 2 presents a more cost-competitive solution for single-phase meters
which allows the same class/range specification to be achieved. This architecture uses a PGA including automatic gain control (AGC) to increase the
AFE input voltage to the required ADC dynamic range, and a ADC with lower performance (16b ENOB) in the current path. The voltage path i
composed of the same 16b ENOB ADC and a PGA with smaller gain values (1-2), as the voltage line variation is usually less than 10 percent. The
digital part allows for phase shifting compensation between the current and voltage path, offset removal and Rogowski coils can interface thanks
to digital integration.
Figure 2. Conventional AFE Used in Electricity Meters
Compared to a high-performance ADC architecture, a PGA and ADC architecture provides many more advantages, including:
Easier implementation.
Reduced area and power consumption.
Evolution to a higher range specification since ViC ADCs with accuracy higher than 21b are not available.
Solutions Expected from a ViC Provider that Broaden the Marketplace for Power Metering SOCs
Nowadays, ViC providers must provide support to SOC integrators developing energy meter SOCs that can compete with standard ICs. The ViC
provider should offer:
A cost-effective solution that occupies a small silicon area and requires few external components.
A high-resolution and low-power solution because of regulatory standards and customer requests for increased dynamic range and accuracy.
A understanding of the application constraints and proper specification of the ViC.
A modular library containing several ADCs, PGAs and voltage references to answer the various needs of power metering solutions and reduce time-to-market.
Solutions that can be used to meet these expectations are discussed in the following sections.
Reducing the Cost of High-Performance Energy Metering ICs: Multiplexed ADC Application to a Three-Phase Energy Meter
Today, the major challenge facing SOC integrators is reducing the cost of energy meter SOCs while maintaining high accuracy. Figure 3 presents two
distinct methods used to achieve a three-phase energy meter with a class of 0.1 and a range of 1/1,000:
A standard solution where currents and voltages are sampled simultaneously via parallel ADCs.
A multiplexed architecture where currents and voltages are sampled using faster ADCs.
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For the past few years, multiplexed successive approximation ADCs have been used in low-end energy meter applications that dont require high
accuracy. When targeting high-end applications that require high accuracy and wide ranges, the modulator becomes incontrovertible. The
multiplexed AFE presented in Figure 3 uses a three-input analog multiplexer, a low-noise PGA with gain steps between 1- 32, and a 16b
modulator with a sampling rate of up to 4kSps. It requires an analog multiplexer frequency of 12 kHz for a measurement bandwidth of 2 kHz. Since
the current in each phase can be different, the embedded AGC evaluates the input signal amplitude and controls the preamplifier gain at each
multiplexer cycle.
Until recently, ADCs have not been considered appropriate for use in high-end applications with multiple multiplexed inputs because they rely
on sinc digital filters which have very slow settling responses. To achieve ADC multiplexing, specific low-latency finite impulse response (FIR
filters are used to overcome the drawbacks of sinc filters and allow ADCs to fully settle on every conversion at rates up to 4kSps. The proposed
multiplexed solution halves power consumption and silicon area compared to the standard three-phase AFE, all the while keeping the accuracyadvantages offered by ADCs.
Figure 3. Standard and Multiplexed AFE Architecture for a Three-Phase Energy Meter
Architectures using a single ADC with a six-input multiplexer for three-phase energy meters are also available. They allow for sequential conversion
of the currents and voltages of the three phases (e.g., phase 1 current, phase 1 voltage, phase 2 current). Nevertheless, the presented architecture
which uses two ADCs offers the following advantages in comparison:
Range specification can be higher since each ADC has only three multiplexed inputs.
Voltage path requirements are less constraining than current path requirements, which allows a lower accuracy ADC to be used for voltage
measurements.
No extra phase shifting is introduced since voltage and current are converted simultaneously.
Easier implementation of the digital part.Advanced Modeling Techniques for Guaranteeing SOC Level Performances
Even if the AFE achieves the required performance, the SOC performance at the system level cannot be guaranteed since AFE performances can be
degraded by poor peripheral components and integration within the rest of the SOC. To guarantee SOC performance and yield, the AFE must be
validated with the peripheral components required by the ADC (clock, reference voltage). To perform this simulation in a short amount of time, an
appropriate electronic design automation (EDA) solution allowing for multi-domain and multi-level simulation; libraries of high-level description
models of electronics (e.g., PGA, ADC and digital-to-analog converter (DACs)); and peripherals (e.g., clock, references, power management and
sensors) are required. In energy meter applications there are four sources of inaccuracies in peripheral components that must be taken into
account to avoid performance degradation:
Clock jitter: Ajittered clock has two or three subsequent periods which are not equal over time. In modulators the input signal is sampled at the
clock frequency, introducing noise in the sampled signal and, consequently, reducing ADC SNR.
Reference voltage noise: The reference voltage gives a clean voltage to the ADC. Its noise level should be specified to avoid an increase of the ADC
noise floor.
Reference voltage temperature drift: ADC gain is sensitive to the voltage reference level which in itself is sensitive to temperature. Since a power
meter should give the same billing in the winter and summer periods, the voltage reference temperature drift must be accounted for to minimize
the ADC gain variation (about 10-50 ppm/C in class 0.1).
Anti-aliasing filter mismatch: The anti-aliasing filters shown in Figure 1 are required in front of any ADC to avoid the aliasing of high-frequency
components present in the power lines. The mismatch value of the RC external components gives a mismatch on the low-pass filter cut-off
frequency and thus a phase error between the voltage and current path. For example, 10 percent-accurate external components can cause a phase
error of about 0.5 and thus an error in the power measurement of about 1.5 percent.
Using this approach provides the means to check that peripheral component specifications are suitable to meet the specific SOC requirements and
highlight possible poor integration.
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Conclusion
Integrating high-performance AFEs in energy meter SOCs is now practicable, but it requires close cooperation between the SOC integrator and ViC
provider to guarantee standard compliance and appropriate yield. A multiplexed architecture integrating a new filter approach overcomes the
performance limitations of conventional modulators, making the AFE for SOC three-phase energy meters more cost-competitive in terms of area
and power consumption.
About the Author
Christian Domingues began his semiconductor career at the Techniques of Informatics and Microelectronics for Integrated Systems Architecture
(TIMA) laboratory, where he worked as an analog research engineer for two years. In 2006, he joined Dolphin integration as an analog engineer,working in the field of high-performance measurement ADCs. Domingues received a masters degree in microelectronics and a Ph.D. in analo
ICs and microelectromechanical systems (MEMS) from the Polytechnical National Institute of Grenoble (INPG) in France in 2001 and 2005
respectively. You can reach Christian Domingues at [email protected].
Pasted from: http://www.design-reuse.com/articles/23746/multiplexed-energy-metering-analog-front-end.html
Energy Measurement ICs Provide Critical MetricsBy Stephen Evanczuk, Electronic Products
Energy harvesting is based on an array of microscale technologies that scavenge power from solar, vibrational, thermal, and biological sources. To
design energy-efficient systems, particularly in the case of solar energy for residential or industrial applications, engineers must be able to measure
energy use via solutions capable of delivering accurate results across a broad range of measurement parameters. This is especially true in
networked systems with multiple nodes, since different nodes may have different harvesting opportunities. In a distributed application, the sameend-user performance may be achieved using different workload allocations and different resultant energy consumptions.
Once largely the sole concern of utility meter designers, accurate measurement of energy use is now available to a broad range of engineers via
highly integrated devices from IC manufacturers, including (alphabetically) Analog Devices, Maxim Integrated Products, NXP
Semiconductors, Renesas,STMicroelectronics, and Texas Instruments.
A typical energy measurement signal chain comprises sensors for current and voltage, an analog front end, analog/digital conversion, and signal
processor along with an optional host processor to handle control, communications, and display functions (Figure 1). In applications requiring
the utmost accuracy, performance, and functionality, designers often need to implement metering solutions using a combination of devices, each
optimized for their role in this signal chain. For many applications, however, integrated devices that combine the entire measurement signal chain
on a single device can offer a sufficiently robust solution.
Figure 1: Engineers can find integrated devices that include on-chip support for each stage of the complete energy-measurement signal chain.
(Courtesy of Analog Devices.)
In its simplest form, engineers can use MCUs with on-chip analog-digital converters (ADC) such as Renesas' H8 MCU (Figure 2). Using a
development kit such as the Renesas H8/38024 Starter Kit and Renesas-provided energy meter software, engineers can quickly implement a single-
phase energy meter.
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Figure 2: Engineers can build a simple energy meter using an MCU with on-chip ADCs and a suitable software library, but depending on the
complexity of the applications, such designs could lead to higher part counts. (Courtesy of Renesas.)
Ensuring reliable measurements can be problematic without special considerations for phase shifts introduced by the sensors collecting the voltage
and current signals. For example, a current transformer can introduce phase errors of 0.1 to 0.3, which must be corrected to ensure accurate
power calculations. Devices built specifically for energy metering typically account for phase errors with on-chip features such as those in the
STMicroelectronics STPM01. The STPM01 allows engineers to digitally calibrate these small phase errors by setting phase compensation values
using the device's 4-bit phase calibration register (CPH).
Designed to measure active, reactive, and apparent power, the STPM01 is a single-chip, mixed-signal device that combines on-chip analog circuitry
measurement and digital circuitry for analysis and control. The analog circuitry includes preamplifier and first-order sigma-delta ADC blocks, band
gap voltage reference, low drop voltage regulator, while the digital subsystem comprises logic for system control, oscillator, hardwired DSP
functionality, and a SPI interface. In standalone operation, the STPM01 provides signals through its power output pins MOP and MON to drive a
stepper motor for controlling a roller counter that could be used to display measurement data.
Other integrated energy-measurement devices such as the Texas Instruments MSP430FE42 offer on-chip circuitry for controlling LCD displays.
Along with a full 16-bit RISC CPU, the TI MSP430FE42x series includes the ESP430CE1 module a specialized on-chip metrology engine. Designed
to provide all the necessary circuitry needed for energy measurement, the ESP430CE1 module combines a hardware multiplier, three independent
16-bit sigma-delta ADCs, and an embedded signal processor, the ESP430.
Analog Devices includes an enhanced 8052 MCU core in its ADE5566 an integrated energy-metering IC that combines the 8052 MCU with fixed-
function DSP functionality, an analog front end and diverse peripherals including an LCD driver required to build a standalone electronic energy
meter. The device's measurement core delivers active, reactive, and apparent energy calculations, as well as voltage and current rms
measurements. In addition, the on-chip DSP functionality supports a variety of power line supervisory functions such as sag, peak, and zero-
crossing measurement typically required in conventional energy meter applications. As with many of these single-chip energy-metering solutions,
kits are available to help engineers evaluate the devices and accelerate new designs.
For its 78M6612 energy measurement IC, Maxim combines a high-performance, 8-bit 8051-compatible MCU with an independent 32-bit
computation engine for performing the various calculations required for energy and power measurement (Figure 3). Also on-chip, the Teridian
Single-Converter Technology measurement engine combines a 22-bit delta-signal ADC, four analog inputs, precision voltage reference, and digital
temperature compensation. Along with 32-Kbyte flash memory, the device includes a comprehensive set of peripherals, including a full
complement of on-chip timing functions, watchdog timer, UART interface, GPIOs, and LCD driver.
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Figure 3: The Maxim 78M6612 collapses the entire energy measurement signal chain onto a single silicon die. In this device, the computational
engine (CE) processes samples from the ADC and communicates with the microprocessor unit (MPU) via status signals and use of shared registers.
(Courtesy of Maxim Integrated Products.)
Maxim's 71M6531 is a system-on-chip (SoC) device intended for residential metering applications. The highly integrated device combines the
features found on the 78M6612 energy measurement IC with additional capabilities such as robust tamper detection typically required in single-
and dual-phase residential metering applications.
NXP offers its own highly integrated SoC for energy metering. The NXP EM773 is an ARM Cortex-M0-based, 32-bit energy-metering IC designed for
smart-metering applications. The device includes 32 Kbyte flash and 8 Kbyte of SRAM for data, 25 GPIO pins, high-current output and sink pins,
three general-purpose timers, programmable watchdog timer, serial interfaces, and multiple clock generation options. Although the EM773 does
not include on-chip LCD drivers as with other devices in this class, NXP designed the device with a bidirectional I2C-bus controller that can be used
for two-way chip-to-chip communications or one-way communications with receiver-only devices such as LCD drivers.
For energy measurement, the EM773 boasts a complete metrology engine that delivers a full range of IEEE Std. 1459-2010 measurements,
including rms voltage, rms current, active power, apparent power, non-active power, power factor, fundamental reactive power, fundamental
apparent power, fundamental power factor, non-fundamental apparent power, and current total harmonic distortion. The device achieves one
percent accuracy while automatically performing calculations without CPU intervention.
As companies respond to increased demand for energy efficiency and try to determine whether the best approach for a given application is
through traditional or alternative-energy means, the need for precise, readily available energy measurement devices grows in importance.
Integrated devices such as those mentioned above offer a cost-effective solution. More information can be found by using the provided links to
product pages on the Digi-Key website.
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